In lactation, the liver synthesizes most of the carbohydrate and some of the lipid that is secreted in milk, and the liver as well as the mammary glands have receptors for prolactin. We have hypothesized that lactation may be regulated in part by a humoral negative feedback system, involving a factor in milk which diffuses from the breast into the plasma and causes inhibition of prolactin binding in the liver and mammary glands, thereby decreasing the response of these organs to prolactin. In investigating this hypothesis, we have found that the s.c. administration of bovine milk to female rats significantly decreases the capacity of the liver cell membranes to specifically bind 125I-ovine prolactin. We propose to: a) attempt to identify the prolactin-binding inhibiting factor (PBIF) in milk; b) examine its mechanism of action; c) determine whether it inhibits prolactin-binding in the lactating mammary gland, and in the mammary carcinomas induced by dimethylbenz(a)anthracene (DMBA) and N-nitrosomethylurea (NMU); d) study the effect of PBIF on the growth of DMBA and NMU-induced mammary carcinomas, which are known to be prolactin-dependent; e) determine whether PBIF is produced by these tumors; and f) observe whether it affects the binding of hormones other than prolactin. There is evidence that NMU produces its carcinogenic effect by methylating the guanine of DNA in the O6 position. In the rat, hypothyroidism decreases the activity of the "DNA repair enzyme" that removes O6-alkylguanine. We have produced moderate hypothyroidism in the rat prior to administration of a single i.v. dose of NMU, by administration of propylthiouracil (PTU), and continued the PTU for an additional 14 days. Surprisingly, 3-1/2 months later the incidence of mammary carcinoma was significantly decreased in the PTU-treated rats compared to controls. We propose to selectively examine the effect of moderate hypothyroidism and hyperthyroidism on the induction phase and on the growth phase of NMU-induced mammary carcinomas, and to investigate the molecular mechanism of any observed effects on induction.